1. Field of the Invention
The present invention relates to systems used to detect nuclear reactor coolant leaks. In particular, the invention relates to the use of sensitivity enhancement devices for allowing airborne particulate monitors to more readily sense reactor coolant leaks.
2. Brief Description of the Prior Art
An airborne particulate radioactivity monitor can be used to detect nuclear reactor coolant leaks by measuring the airborne particulate activity produced by the leak. A problem with these prior art detectors, however, is the hard-to-quantify losses of radioactive particulates, occurring both at the leak location and as a result of plate-out losses in the sample line between the leak location and the monitoring location.
Gaseous radionuclides have nearly a 100% release to the air from nuclear reactor coolant leaks and have minimal plate-out losses in the sample line. Gas airborne radioactivity monitors have much lower sensitivity than particulate airborne radioactivity monitors, however, because the gas monitor only responds to the quantity of gas in the sample chamber viewed by the detector. The particulate monitor, on the other hand, responds to the radioactive particulates that have been collected over an extended period of time on a filter. The ratio of sensitivities for particulate to gaseous monitors is on the order of 10,000.
In summary, both the particulate monitor and the gas monitor have serious drawbacks when used for nuclear reactor coolant leak detection. The particulate monitor has hard-to-quantify loss of particulate activity, both at the leak location and losses resulting from plate-out on the sample line walls. The gas monitor has extremely low sensitivity compared to the particulate monitor.
U.S. Pat. No. 4,820,925, discloses a room air monitor for radioactive aerosols that uses a filter system to collect suspended particles that are carried through the system through the airflow into the monitor, and these particles are detected and examined for radioactivity by the detector-preamplifier combination of the device. This device could not, by itself, be used to examine the radioactivity due to gaseous radionuclides escaping from a nuclear power plant.
U.S. Pat. No. 3,712,850, discloses a method for determining reactor coolant system leakage that uses a gas detector to measure the radioactivity produced by short half-lived noble gas isotopes. This invention suffers from the previously noted insensitivity of gaseous monitors.
U.S. Pat. No. 4,092,539, discloses a system for indicating radiation from a radioactive fluid such as a gas, wherein simultaneous indications of the activity concentration of radioactivity of the gas, the radiation dose rate, and average energy of the radiation are provided.
U.S. Pat. No. 4,262,203, discloses an alpha particle monitor which includes a flow-through linear ionization chamber with means to pass therethrough gas-borne radioactive matter exhibiting alpha decay, such as radon gas, airborne plutonium or uranium dust.
U.S. Pat. No. 4,426,581, discloses an arrangement for measuring radioactivity concentrations. The apparatus measures noble gas activity concentration and aerosol activity concentration and compresses the gas whose activity concentration is to be measured in order to reduce the volume of the measuring chamber and the weight of the shielding housing.
U.S. Pat. No. 3,621,238, discloses a gamma insensitive air monitor for radioactive gases. This disclosure utilizes an ionization chamber instrument for measuring the concentration of the radioactive gases in the air. External gamma radiation effects on the instrument are eliminated through the use of a compensating chamber mounted next to the measuring chamber, with both chambers orbiting about a common axis.
U.S. Pat. No. 3,849,655, discloses a light water reactor primary coolant leakage monitor. The monitor relies on the radioactive decay of nitrogen-13 and fluorine-18 found in irradiated water, by the emission of positrons.
Despite the above-described devices and methods, there remains a need in the art for a reliable method of measuring airborne radioactivity resulting from nuclear reactor coolant leaks.